As is known, a catalyst becomes effective only above a minimum operating temperature. Below this minimum operating temperature, the catalyst is not able to convert the noxious substances contained in the exhaust gas of an internal combustion engine into less noxious substances. For the smallest possible emission of pollutants, the catalyst should be ready for operation during the entire time during which pollutants are present, i.e., during the entire time between the starting and stopping of the internal combustion engine. Since heating of the catalyst is effected by the exhaust gases of the internal combustion engine--and possibly also, for instance, by additional electric heating--the catalyst is necessarily below the minimum operating temperature upon starting of the internal combustion engine and, in part also, under certain operating conditions of the internal combustion engine, such as long-continued coasting. In order to achieve the highest possible availability of the catalyst, it is thus advisable to heat the catalyst. There are certain problems, however, associated with heating the catalyst, and in particular excessively high temperatures can result in damage to the catalyst, and heating causes an additional consumption of energy or an increased emission of pollutants, reduced driving comfort, and reduced power.
An apparatus for heating a catalyst in the exhaust system of an internal combustion engine is described in U.S. Pat. No. 4,165,610 to Iizuka et. al. (the "Iizuka patent"). In that apparatus, fuel is injected into the engine via an additional injection nozzle when the temperature of the catalyst drops below the minimum operating temperature. It is stated in the Iizuka patent that upon a cold start or as a result of stalling in the lower load range, the temperature of the catalyst could be below the minimum operating temperature.
U.S. Pat. No. 4,622,809 to Abthoff et. al. describes how the cracking property of a catalyst can be determined from the amplitude of the output signal of an oxygen sensor which is arranged downstream of the catalyst.